With the recent trend toward size reduction in electronic appliances, secondary batteries are increasingly required to have a higher capacity, etc. Attention is hence focused on lithium secondary batteries, which have a higher energy density than nickel-cadmium batteries and nickel-hydrogen batteries.
The electrolytes used in lithium secondary batteries are nonaqueous electrolytes prepared by dissolving an electrolyte such as LiPF6, LiBF4, LiClO4, LiCF3SO3, LiAsF6, LiN(CF3SO2)2, or LiCF3(CF2)3SO3 in a nonaqueous solvent such as acyclic carbonate, e.g., ethylene carbonate or propylene carbonate, a chain carbonate, e.g., dimethyl carbonate, diethyl carbonate, or ethyl methyl carbonate, a cyclic ester, e.g., γ-butyrolactone or γ-valerolactone, a chain ester, e.g., methyl acetate or methyl propionate, or the like.
<Nonaqueous Electrolytes 1 and 1-1, Nonaqueous-Electrolyte Secondary Batteries 1 and 1-1>:
First, such nonaqueous-electrolyte secondary batteries have advantages of having a high energy density and being less apt to suffer self-discharge. In recent years, the secondary batteries are hence extensively used as power sources for mobile appliances for public use, such as portable telephones, notebook personal computers, and PDAs. The electrolytes for nonaqueous-electrolyte secondary batteries are constituted of a lithium salt as a supporting electrolyte and a nonaqueous organic solvent. The nonaqueous organic solvent is required to have a high permittivity for dissociating the lithium salt, to show high ionic conductivity in a wide temperature range, and to be stable in the batteries. It is difficult to meet these requirements with a single solvent. Because of this, use is generally made of a combination of a high-boiling solvent represented by propylene carbonate, ethylene carbonate, or the like and a low-boiling solvent such as dimethyl carbonate or diethyl carbonate.
On the other hand, many reports have been made on the addition of various additives to electrolytes in order to improve initial capacity, rate characteristics, cycle performances, high-temperature storability, continuous-charge characteristics, self-discharge characteristics, overcharge-preventive properties, etc. For example, addition of a lithium fluorophosphate compound has been reported as a technique for inhibiting self-discharge at high temperatures (see patent document 1).
<Nonaqueous Electrolyte 2 and Nonaqueous-Electrolyte Secondary Battery 2>:
Secondly, various investigations have been made on nonaqueous solvents and electrolytes in order to improve the battery characteristics including output characteristics, cycle performances, and storability of those lithium secondary batteries. For example, patent document 2 describes a technique in which a battery having excellent low-temperature output characteristics is produced by using an electrolyte containing a tetrafluoroboric acid salt in a certain amount relative to the overall area of the active-material layer formed on the positive-electrode current collector.
This technique has, in some degree, the effect of improving output characteristics without reducing high-temperature cycle performances. However, the degree of output improvement attainable with this technique is limited, and the technique failed to attain an even higher output.
<Nonaqueous Electrolyte 3 and Nonaqueous-Electrolyte Secondary Battery 3>:
Thirdly, various investigations have been made on nonaqueous solvents and electrolytes in order to improve the battery characteristics including load characteristics, cycle performances, storability, and low-temperature characteristics of those lithium secondary batteries. For example, patent document 3 includes a statement to the effect that when an electrolyte containing a vinylethylene carbonate compound is used, the decomposition of this electrolyte is minimized and a battery excellent in storability and cycle performances can be fabricated. Patent document 4 includes a statement to the effect that when an electrolyte containing propanesultone is used, recovery capacity after storage can be increased.
The incorporation of such compounds can produce, in some degree, the effect of improving storability and cycle performances. However, those techniques have had a problem that a coating film having high resistance is formed on the negative-electrode side and this, in particular, reduces discharge load characteristics.
<Nonaqueous Electrolyte 4 and Nonaqueous-Electrolyte Secondary Battery 4>:
Fourthly, various investigations have been made on nonaqueous solvents and electrolytes in order to improve the battery characteristics including load characteristics, cycle performances, storability, and low-temperature characteristics of those lithium secondary batteries. For example, patent document 3 includes a statement to the effect that when an electrolyte containing a vinylethylene carbonate compound is used, the decomposition of this electrolyte is minimized and a battery excellent in storability and cycle performances can be fabricated. Patent document 4 includes a statement to the effect that when an electrolyte containing propanesultone is used, recovery capacity after storage can be increased.
The incorporation of such compounds produces, in some degree, the effect of improving storability and cycle performances. However, those techniques have had a problem that a coating film having high resistance is formed on the negative-electrode side and this, in particular, reduces discharge load characteristics.
On the other hand, it has been reported in patent document 5 that the addition of a compound represented by the formula (1) given in patent document 5 improves both cycle performances and current characteristics. It has also been reported in patent document 6 that the addition of a specific compound improves low-temperature discharge characteristics.
However, battery characteristics such as load characteristics, cycle performances, storability, and low-temperature characteristics are still insufficient, and there has been room for improvement.
<Nonaqueous Electrolyte 5 and Nonaqueous-Electrolyte Secondary Battery 5>:
Fifthly, nonaqueous-electrolyte batteries including lithium secondary batteries are coming to be practically used in extensive applications ranging from power sources for applications for public use, such as, e.g., portable telephones and notebook personal computers, to on-vehicle power sources for driving motor vehicles or the like. However, recent nonaqueous-electrolyte batteries are increasingly required to have higher performances, and there is a desire for improvements in both battery characteristics and battery safety.
Electrolytes for use in nonaqueous-electrolyte batteries are usually constituted mainly of an electrolyte and a nonaqueous solvent. As main components of the nonaqueous solvent, use is being made of: cyclic carbonates such as ethylene carbonate and propylene carbonate; chain carbonates such as dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate; cyclic carboxylic acid esters such as γ-butyrolactone and γ-valerolactone; and the like.
However, these organic solvents have volatility and are apt to catch fire. Because of this, nonaqueous-electrolyte batteries employing an electrolyte containing any of those organic solutions in a large amount potentially have the risk of igniting or exploding in case where the batteries are misused or improperly used, for example, the batteries are heated, suffer internal short-circuiting or external short-circuiting, or are overcharged or overdischarged, or in case of an accident. Such risk is exceedingly high in large batteries intended to be used especially as power sources for motor vehicles.
From such standpoints, a nonaqueous electrolyte containing an ambient-temperature-molten salt (also called room-temperature-molten salt or ionic liquid) has been proposed. Although liquid, this ambient-temperature-molten salt is too low in volatility to be detected. It is also known that this salt does not burn because it does not volatilize. Patent document 7 discloses an attempt to obtain a nonaqueous-electrolyte battery having excellent safety by using the ambient-temperature-molten salt as an electrolyte for lithium secondary batteries.
Furthermore, patent document 8 discloses a technique in which an ambient-temperature-molten salt having a quaternary ammonium cation and having excellent reductional stability is dissolved in combination with a compound, such as ethylene carbonate or vinylene carbonate, which undergoes reductional decomposition at a nobler potential than the ambient-temperature-molten salt. According to this technique, the compound which undergoes reductional decomposition at a nobler potential than the ambient-temperature-molten salt electrochemically reacts in the step of initial charge/discharge to form an electrode-protective coating film on the electrode active materials, in particular, on the negative-electrode active material, to thereby improve charge/discharge efficiency.    Patent Document 1: Japanese Patent No. 3439085    Patent Document 2: JP-A-2004-273152    Patent Document 3: JP-A-2001-006729    Patent Document 4: JP-A-10-050342    Patent Document 5: JP-A-08-078053    Patent Document 6: JP-A-11-185804    Patent Document 7: JP-A-4-349365    Patent Document 8: JP-A-2004-146346